1. Field of the Invention
The present invention relates to a mounting structure for mounting electronic components such as a semiconductor device, a method for producing the same, and an electrically conductive adhesive used for production of the mounting structure.
2. Description of the Related Art
Heretofore, in mounting a semiconductor device onto input/output terminal electrodes of a circuit board, one often utilizes the wire bonding method by soldering. In recent years, however, owing to scale reduction of a package of semiconductor devices and increased number of connection terminals, an interval between the connection terminals is becoming smaller, making it more and more difficult to perform the mounting with the use of a conventional soldering technique.
Therefore, in recent years, a method is proposed in which the semiconductor device such as an integrated circuit chip is directly mounted on the input/output terminali electrodes of the circuit board for scale reduction and effective utilization of a mounting area.
Above all, a flip-chip mounting method in which a semiconductor device is mounted facedown on a circuit board is regarded as being a useful method because a collective electrical connection can be established between the semiconductor device and the circuit board in addition to large mechanical strength after connection.
As a connecting material for the aforesaid flip-chip mounting method, a system using an electrically conductive adhesive is proposed or put into practical use.
This method is a promising method in view of both an improvement in reliability and a measure for environmental protection, due to the following two reasons.
Firstly, since the electrically conductive adhesive contains a resin material such as an epoxy resin, a soft connection is established against external force or thermal stress as compared with a solder, i.e. a metal material, thereby leading to an improvement in reliability.
Secondly, since the electrically conductive adhesive mainly contains particles of silver as an electrically conductive component, a clean mounting without the use of lead can be carried out.
On the other hand, for a conventional structure in which an electronic component such as a chip component or a package component is mounted onto a printed board, a structure is likewise proposed in which an electrically conductive adhesive is used in view of an improvement in reliability and a measure for environmental protection.
Thus, the mounting structure using an electrically conductive adhesive, will be a promising method in view of both an improvement in reliability and a measure for environmental protection in the future.
However, the mounting structure using an electrically conductive adhesive has the following two problems, thereby hindering the practical use thereof.
The first problem is a decrease in reliability of insulation caused by ion migration. The ion migration is a kind of electrolytic action, and is a phenomenon of insulation breakdown that occurs between electrodes through the following four steps when an electrolyte such as water is present between the electrodes to which a voltage is applied.
Step 1) Elution and ionization of positive electrode metal
Step 2) Migration of the ionized metal towards the negative electrode by application of a voltage
Step 3) Deposition of the metal ion migrated to the negative electrode
Step 4) Repetition of steps 1) to 3)
Through such an ion migration phenomenon, the metal grows like a dendrite between the electrodes and finally the electrodes are bridged with the metal to cause an insulation breakdown.
Silver used as a material of an electrically conductive filler of an electrically conductive adhesive has a tendency of being eluted, and is liable to cause ion migration. Further, in accordance with the further reduction in scale and weight of electronic apparatus in recent years, the pitch of electrodes disposed on a semiconductor device, an electronic component, or a circuit board is becoming further smaller, thereby all the more increasing the likelihood of ion migration. For this reason, it is essential to solve the problem of ion migration for practically carrying out the mounting with an electrically conductive adhesive.
Conventionally, for restraining the ion migration, the following three methods are proposed:
Proposal 1) Forming an electrically conductive filler with an alloy (silver-copper, silver-palladium, or the like),
Proposal 2) Sealing an electrically conductive adhesive with an insulating resin such as an epoxy resin,
Proposal 3) Capturing eluted metal ions and forming them into an insoluble substance by addition of an ion-capturing agent such as an ion-exchange resin or a chelating agent to an electrically conductive adhesive.
However, these proposals involve the following disadvantages. In the proposal 1, the filler metal will be extremely expensive, thereby increasing the costs for producing the electrically conductive adhesive. In the proposal 2, addition of the sealing step necessitates an increase in the number of steps and extensive installation of additional equipment, thereby raising the production costs. In the proposal 3, elution of metal ions from the electrically conductive filler decreases the contact property of the electrically conductive filler to raise the connection resistance.
Thus, though the aforementioned proposals produce an effect of restraining the ion migration, they involve other problems, so that it is difficult to put these proposals into practical use except for special fields. The second problem is a rise in the connection resistance by sulfurization. Sulfurization is a phenomenon in which a metal changes to a substance called metal sulfide having a lower electrical conductivity by reacting with a weakly acidic gas containing sulfur, such as hydrogen sulfide or sulfur dioxide. The sulfurization seems to occur through the following steps, though a lot of portions thereof still remain unresolved.
Step 1) Elution and ionization of a metal in a weakly acidic atmosphere
Step 2) Generation of metal sulfide by reaction of sulfur ions and metal ions
As described previously, the electrically conductive filler is mainly composed of silver as a major component. However, since silver is extremely liable to be sulfurized, the volume resistivity of the electrically conductive adhesive increases when silver is sulfurized and, in accordance therewith, the connection resistance rises. Currently, few solutions to this problem have been reported, and it has been impossible to apply a mounting structure using an electrically conductive adhesive to a product of electronic components that may be possibly used in an environment, such as the neighborhood of a spa or a volcano, where hydrogen sulfide or sulfur dioxide is present at a relatively high concentration. For this reason, the field of applying the mounting structure using an electrically conductive adhesive has been extremely limited.
Thus, a principal object of the present invention is to maintain reliability of a mounting structure using an electrically conductive adhesive even under a comparatively severe condition such as a condition of much humidity or a gas atmosphere containing sulfur.
In order to achieve the aforesaid object, the present invention provides, in short, a mounting structure comprising an electric structure and an electrically conductive adhesive layer disposed on said electric structure, wherein said electrically conductive adhesive layer contains an electrically conductive filler, and an elution preventive film is disposed on at least a part of said electrically conductive filler. Owing to this construction, the mounting structure of the present invention exhibits a good insulation reliability even under a high temperature and high humidity environment. This is because the elution preventive film prevents elution of metal in the electrically conductive filler even under a high temperature and high humidity condition, thereby fundamentally preventing the ion migration reaction. Further, the mounting structure of the present invention does not cause sulfurization of an electrically conductive filler and shows a good connection reliability even if it is left to stand in a gas containing sulfur. This is because the sulfurization reaction can be fundamentally prevented by prevention of the elution of the electrically conductive filler. The elution preventive film used in the present invention may be made, for example, of a thermosetting resin, a thermoplastic resin, a metal alkoxide, or the like. Specific examples of the thermosetting resin include phenolic resin, urea resin, melamine resin, furan resin, unsaturated polyester resin, diallyl phthalate resin, epoxy resin, silicone resin, polyimide resin, and others. Specific examples of the thermoplastic resin include vinyl chloride resin, vinylidene chloride resin, polystyrene, AS resin, ABS resin, methacrylic resin, polyethylene, ionomer, methylpentene resin, polyallomer, fluororesin, polyamide, polyimide, polyamideimide, polyurethane, polycarbonate, polyester, polyacetal, denatured polyphenylene oxide, polysulfone, polyphenylene sulfide, and others. Specific examples of the metal alkoxide include silicon tetraethoxide, aluminum tributoxide, titanium tetrabutoxide, and others.
Here, if the mounting structure further comprises another electric structure disposed on said electric structure and said electrically conductive adhesive layer electrically connects said electric structure to said other electric structure, then the present invention will be especially effective, because the effect of ion migration reaction or sulfurization of the electrically conductive filler on the connection resistance will be considerable.
Specifically, it is sufficient if at least a part of said electrically conductive filler is exposed on a surface of said electrically conductive adhesive layer, and said elution preventive film is disposed at least on said exposed part of said electrically conductive filler. More specifically described, it is sufficient if said electrically conductive adhesive layer has numerous holes that are in communication with each other and with a surface of said electrically conductive adhesive layer; at least a part of said electrically conductive filler is exposed to an inner surface of said holes; and said elution preventive film is disposed at least on said electrically conductive filler that is exposed to said inner surface of said holes. This construction allows an elution preventive film to be selectively formed only on a portion of the electrically conductive filler which is in communication with the surface of the electrically conductive adhesive layer and which is liable to be eluted. This provides the following advantage.
If an elution preventive film is disposed on a portion of the filler that is involved in electrical connection, the elution preventive film in some cases causes an obstacle against electric conduction. Therefore, if the elution preventive film is present on this portion, a treatment such as pressing it along the conduction direction from outside is carried out to break the elution preventive film on this portion to ensure the electric conduction. In contrast, if the elution preventive film is selectively disposed only on a portion where the elution is liable to occur, there will be only a limited amount of the elution preventive film on a portion of the filler where the electric conduction must be established, thereby all the more improving the electric conduction.
Further, the elution preventive film is preferably water-insoluble, because then the elution preventive film will not elute even under a high temperature and high humidity condition, and the decrease in the elution preventing effect will be less liable to occur, whereby the effect of restraining the ion migration reaction and the effect of restraining the sulfurization of the electrically conductive filler can be maintained for a long period of time.
The water-insoluble elution preventive film that can be used in the present invention may be made, for example, of a thermosetting resin, a thermoplastic resin, a metal alkoxide, or the like. Specific examples of the thermosettin resin include phenolic resin, urea resin, melamine resin, furan resin, unsaturated polyester resin, diallyl phthalate resin, epoxy resin, silicone resin, polyimide resin, and others. Specific examples of the thermoplastic resin include vinyl chloride resin, vinylidene chloride resin, polystyrene, AS resin, ABS resin, methacrylic resin, polyethylene, ionomer, methylpentene resin, polyallomer, fluororesin, polyamide, polyimide, polyamideimide, polycarbonate, denatured polyphenylene oxide, polyphenylene sulfide, and others. Specific examples of the metal alkoxide include silicon tetraethoxide, aluminum tributoxide, titanium tetrabutbxide, and others.
The elution preventive film is preferably insoluble in an aqueous solution containing hydrogen sulfide or sulfur oxide, because then the effect of preventing the sulfurization reaction will be larger. This is due to the fact that, even if hydrogen sulfide or sulfur oxide is condensed as an aqueous solution on the surface of the elution preventive film, the elution preventive film does not elute, so that decrease in the elution preventive effect is unlikely to occur.
The elution preventive film insoluble in an aqueous solution containing hydrogen sulfide or sulfur oxide maybe made, for example, of a thermosetting resin, a thermoplastic resin, a metal alkoxide, or the like. Specific examples of the thermosetting resin include phenolic resin, melamine resin, furan resin, unsaturated polyester resin, diallyl phthalate resin, epoxy resin, silicone resin, polyimide resin, and others. Specific examples of the thermoplastic resin include vinyl chloride resin, vinylidene chloride resin, polystyrene, As resin, ABS resin, methacrylic resin, polyethylene, ionomer, methylpentene resin, polyallomer, fluororesin, polyamide, polyimide, polyamideimide, polycarbonate, denatured polyphenylene oxide, polyphenylene sulfide, and others. Specific examples of the metal alkoxide include silicon tetraethoxide, aluminum tributoxide, titanium tetrabutoxide, and others.
Here, the term xe2x80x9cinsolubility in an aqueous solution containing hydrogen sulfide or sulfur oxide or in waterxe2x80x9d is used to mean that it satisfies the following conditions. Namely, if the elution preventive film is formed of a resin, the aforesaid insolubility is defined as a property such that the amount of absorbed water or aqueous solution after 24 hours is less than or equal to 0.5 wt %. If the elution preventive film is formed of a complex, the aforesaid insolubility is defined as a property such that its solubility (weight of the complex dissolved in 100 g of water) is less than 1xc3x9710xe2x88x925 g.
The elution preventive film preferably contains a metal complex. The metal complex is a coordination compound produced by reaction of a metal with a complexing agent. This reaction proceeds rapidly at room temperature. Therefore, formation of a complex film on the surface of an electrically conductive filler is carried out by allowing the electrically conductive filler to contact with a complexing agent. The metal complex formed on the filler surface has an extremely strong coordination bond between the metal and the complexing agent, so that the metal complex is stable and has an excellent close contact with the metal. This, increases the elution preventing effect further, thereby producing a better effect of improvement in anti-ion-migration property and a better effect of improvement in anti-sulfurization-reaction property. Here, the ligand of the complexing agent for forming the metal complex may be, for example, aminoacetyl group, aminocarboxyl group, alkanolamine group, xcex2-diketone group, xcex2-ketoester group, polyamine group, imidazole group, or the like. Among these, use of aminocarboxyl group or imidazole group,bonded to a benzene ring is preferable because then the metal complex film will be insoluble in water and insoluble in an aqueous solution containing hydrogen sulfide or sulfur oxide, thereby further increasing the effect of restraining the ion migration and the effect of restraining the sulfurization reaction. Examples of such complexing agents include anthrahilic acid, 2-aminopiperidine, galloylgallic acid, potassium xanthate, oxine, quinaldinic acid, cupferron, 4-chloro-3-methyl-5-nitrobenzensulfonic acid, salicylaldoxime, diantipyrylmethane, diethyldithiocarbamic acid, p-dimethylaminobenzylidenerhodanine, dimethylglyoxime, oxalic acid, cinchonine, N-cynnamoyl-N-phenylhydroxylamine, thioacetamide, thionalide, thiourea, tetraphenylboric acid, trimethylphenylammonium, 1-nitroso-2-naphthol, nitron, neocupferron, bismuththiol II, p-hydroxyphenylarsonic acid, 8-hydroxy-7-iodo-5-quinolinesulfonic acid, pyrogallol, 1-pyrrolidinecarbodithioic acid, phenylarsonic acid, phenylthiodantoic acid, phenylfluorone, xcex1-furildioxime, brucine, benzidine, N-benzoyl-N-phenylhydoxylamine, xcex1-benzoinoxime, benzo[f]quinoline, 2-mercaptobenzothiazole, rhodamine B, and others.
Furthermore, the elution preventive film is not limited to a metal complex alone, and it may further contain a resin component. In short, it is sufficient if the elution preventive film has a function capable of preventing elution.
Here, if the electrically conductive filler contains silver, the effect of the present invention, that is, the effect of restraining the ion migration and the effect of restraining the sulfurization, will be conspicuous, because silver is a substance liable to cause ion migration an sulfurization reaction.
Examples of the aforesaid mounting structures to which the present invention can be applied include a circuit board having electronic components mounted thereon, a circuit board having a semiconductor device flip-chip mounted thereon, and others.
An adhesive of the present invention contains an electrically conductive filler, and at least a part of the electrically conductive filler has an elution preventive film. By this construction, a mounting structure mounted with the use of an electrically conductive adhesive of the present invention exhibits a good insulation reliability without causing ion migration even under a high temperature and high humidity environment. Further, it exhibits a good connection reliability without causing sulfurization of the electrically conductive filler even if it is left to stand in a gas containing sulfur.
Here, regarding the composition of the elution preventive film, it is preferably water-insoluble, it is preferably further insoluble in an aqueous solution containing hydrogen sulfide or sulfur oxide, it preferably contains a metal complex, and it preferably contains a resin component. Furthermore, the electrically conductive adhesive preferably contains silver.
Also, the present invention provides a method of producing a mounting structure, said method comprising the steps of forming an electrically conductive adhesive layer on an electric structure, said electrically conductive adhesive layer containing an electrically conductive filler; and forming an elution preventive film on said electrically conductive filler that is in communication with a surface of said electrically conductive adhesive layer after said electrically conductive adhesive layer is hardened. The mounting structure produced by this production method can exhibit an effect of improving the anti-ion-migration property and an effect of improving the anti-sulfurization-reaction property while maintaining the electric conduction of the electric structure well by forming the elution preventive film. Further, since the elution preventive film is formed on the electrically conductive filler after the electrically conductive adhesive layer is hardened, the elution preventive film does not enter an electric connection site of the electrically conductive adhesive layer to produce an adverse effect on the electric conduction state thereof, so that a good electric conduction can be ensured.
One method for forming an elution preventive film in this manner is a process of forming a complex. The process of forming a complex can perform the following function. The complexing process is carried out, for example, by pouring a complexing agent. Such a complexing process can be carried out with the use of an already-existing equipment, thereby eliminating the need for a new investment on equipment and inviting little increase in the production costs. Here, if a complex film is formed on a contact point between electrically conductive fillers or on a contact point between an electrically conductive filler and an electrode metal, it may possibly deteriorate the electric conductivity of the electrically conductive adhesive and the electric characteristics of the mounting structure to some extent, since complexes are generally insulating. In contrast, if the production method of the present invention is used, the electrically conductive filler is formed into a complex after the electrically conductive adhesive layer is hardened to ensure its electric conduction, so that only the site which is not involved in electric conduction can be formed into a complex while maintaining the aforesaid contact point between the electrically conductive fillers and the contact point between the electrically conductive filler and other electric structures electrically connected to the electrically conductive adhesive layer, as they are. Therefore, the elution preventive film can be effectively formed while ensuring the electric characteristics of the mounting structure, thereby giving a low connection resistance. Here, in this process, since the complexing agent reacts selectively to the metal portions, the elution preventive film is not formed on a site where the complexing process is not required, that is, the site of a resin component (binder resin or the like) of the electrically conductive adhesive layer or the surface of the electric structure (surface of the board or the like). Further according to the method of the present invention, since the complexing process is carried out after fabricating a mounting structure using an arbitrary electrically conductive adhesive, the present invention can be applied to a mounting structure that uses any kind of an electrically conductive adhesive, so that its economic effect is large.
Specific examples of mounting structures that can be produced by application of the present invention include a mounting structure in which another electric structure is electrically connected to said electric structure by said electrically conductive adhesive layer, a mounting structure in which an electronic component is mounted on a circuit board, and a mounting structure in which a semiconductor device is flip-chip mounted on a circuit board. In particular, if the present invention is applied to a mounting structure in which electronic components made of chip components such as a chip resistor are mounted on a circuit board, it can perform the following function.
If the aforementioned electronic components are mounted, their connection sites are exposed, so that an elution preventive film can be formed on the connection sites of all the components at the same time by a process such as a complexing process on the mounting structure after completion of the mounting structure. Therefore, the period of time required in forming an elution preventive film can be shortened. Here, if the elution preventive, film is formed by a complexing process, the complex film (elution preventive film) is not formed on a portion that need not be made into a complex, that is, the surface of the board, the surface of the components, or the like, because the complexing agent reacts selectively with a metal portion.